The present invention relates generally to a die apparatus and method for high temperature forming of metal products, also known as superplastic forming (SPF).
U.S. Pat. No. 5,823,034 describes a die apparatus for high temperature forming of metal parts which comprises two or more die segments of a suitable non-metallic material to form a die cavity of predetermined shape matching that of the desired part. A sheet metal blank, commonly in the form of two face-to-face sheets welded together around most of their perimeter, is positioned between the die parts and the die is then closed. The sheet metal blank is heated in the die and expanded by blowing gas into the space between the sheets, so that the heat-softened sheets superplastically expand outward and conform to the interior surface of the die. The gas is then relieved and the molded part is allowed to cool, after which the die is opened and the part is removed.
The forming die for this method is generally made of carbon/graphite, which has a relatively low coefficient of thermal expansion. The sheet metal to be formed has a relatively high coefficient of thermal expansion (CTE). As the temperature is increased, the sheet metal will expand and take the form of the forming die. Upon completion of the forming, when the assembly is cooled, the sheet metal will shrink more than the die, due to the difference in CTE between the two materials. With proper design, the sheet metal will shrink away from shapes and features in the die. This die apparatus and method is therefore adequate for simple shapes and those that do not have indentations or shaped features on the surface of the formed product. However, as new shapes are conceived for the sheet metal forming process, limitations are being reached for this basic technique to avoid binding or locking of the shaped part onto the formed features in the die. With more complex shapes, shrinkage of the sheet metal part as it cools at a faster rate than the die may cause it to lock onto features or projections in the die, damaging the forming die and making it unusable for further sheet metal forming.